Facsimile (Fax) machines "read" a document by scanning a light across document and detecting the intensity of the reflections. Feeding a document into a Fax machine to be scanned for transmission requires the use of two sensors. The first sensor determines if the user has placed a document into the document feeder. After tripping the first sensor, a second sensor is required to find the leading edge of that document so that the scanner can be turned on at the appropriate time.
If the scanner is turned on too early, before the edge of the document is over the scanner, a black shadow will appear in the scanned data due to the light reflecting from the edge of the document and being diverted from the scanner. This shadow is objectionable to the user. If the scanner is turned on too late, then any writing near the top of the document will be lost having never been scanned. The goal then is to scan as close to the leading edge of the document as possible without scanning the black shadow.
Traditional solutions to this problem have incorporated optical slot interrupter type sensors mounted on their own small printed circuit assembly (PCA). This assembly is then connected to a main PCA with a wiring harness/connector assembly. The optical sensors have mechanical "flags" which move as the paper travels through the paper path of the Fax. When a flag moves, it causes the optical sensor to change state signifying the paper is in a particular position in the Fax machine.
The leading edge sensor flag is positioned in the Fax machine such that when the leading edge of the document causes the flag to move, the document is in the proper position for the document scan to begin. While useable, this technique of detecting when the scan process should begin has many shortcomings. For example, the separate assembly is costly both in terms of material costs and in the additional labor required to assemble the Fax machine. Also the mechanical tolerance "stack-up" associated with the sensor assembly and its mounting does not lend itself to a small beginning scan margin. The beginning scan margin is the distance between the leading edge of the document and the line at which writing can be seen by the scanner. Because of the stack-up problems, Fax machines typically utilize a wide scan margin of about 4 millimeters (mm). Therefore any printing closer to the leading edge of the paper than 4 millimeters will not be scanned.
What is needed in the industry is a method of eliminating the need for a separate PCA for the paper sensors and for reducing the scan margin below 4 mm so that more of the document page can be utilized.